Method of producing a bonded wafer and the bonded wafer

ABSTRACT

There is provided a method of producing a bonded SOI wafer wherein a silicon single crystal ingot is grown according to Czochralski method, the single crystal ingot is then sliced to produce a silicon single crystal wafer, the silicon single crystal wafer is subjected to heat treatment in a non-oxidizing atmosphere at a temperature of 1100° C. to 1300° C. for one minute or more and continuously to a heat treatment in an oxidizing atmosphere at a temperature of 700° C. to 1300° C. for one minute or more without cooling the wafer to a temperature less than 700° C. to provide a silicon single crystal wafer wherein a silicon oxide film is formed on the surface, and the resultant wafer is used as the bond wafer, and a bonded SOI wafer produced by the method. There can be provided a SOI wafer that has a SOI layer having few crystal defects, good surface roughness and high quality in high productivity, in high yield and with low cost.

This is a Division of application Ser. No. 09/830,389 filed Apr. 26,2001 U.S. Pat. No. 6,492,682. The entire disclosure of the priorapplication is hereby incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present invention relates to a method of producing a bonded waferhaving very few crystal defects on and near the surface and a bonded SOIwafer.

BACKGROUND ART

SOI (silicon on insulator) has a buried oxide film (BOX: Buried OXide)as a insulator film right below a silicon layer that is to be a regionfor fabrication of device, and is expected to be a silicon material forhigh speed device with electric power saving performance. One of methodsfor producing a SOI wafer is a bonding method wherein two silicon singlecrystal wafers, one of which is to be a bond wafer (a substrate that isto be a SOI layer on which a device is fabricated), and the other ofwhich is to be a base wafer (a substrate supporting the SOI layer) arebonded via a oxide film, and then thickness of the bond wafer isdecreased to form a SOI structure. The method has an advantage thatcrystallinity of the SOI layer is excellent, and insulating property ofBOX is high, but has a disadvantage that quality of the SOI layer isinfluenced much by quality of the bond wafer.

Specifically, it has been known that there exist micro crystal defects(Grown-in defects) such as COP (Crystal Originated Particles) that is avoid type defect or the like in a silicon wafer produced according toCzochralski method, which adversely affects characteristics of thedevice such as oxide dielectric breakdown voltage. In order to solve theproblem, there have been known that visible defects can be reduced byusing, as a substrate for a bond wafer, a wafer wherein a CZ wafer issubjected to annealing in a hydrogen atmosphere or an epitaxial waferwherein an epitaxial layer is formed on a CZ wafer (See Japanese PatentApplication Laid-open (kokai) No. 9-22993 and Japanese PatentApplication Laid-open (kokai) No. 9-260619).

However, two heat treatments, namely heat treatment such as hydrogenannealing or epitaxial growth and heat treatment for forming a buriedoxide film, which may lead to increase of cost and lowering of throughput.

In the case of the epitaxial wafer, haze (surface roughness) isgenerated on the surface of the epitaxial layer, or projection calledmound is sometimes formed. They may cause bonding failure when thewafers are bonded. Accordingly, it is sometimes necessary to polish thesurface of the epitaxial layer before bonding in that case.

On the other hand, crystal defects are reduced by hydrogen annealingonly at layer quite near the surface (about 0.5 μm from the surface),and thus, if a SOI wafer having a thickness more than the value isproduced, an area where crystal defects are not reduced is exposed.Therefore, crystal defects in the whole SOI layer cannot be reduced,unless any measures are taken, for example, further hydrogen annealingis conducted after SOI wafer is produced. Furthermore, according toannealing with hydrogen, quartz tube, a boat made of SiC or the like arealways etched, and contamination with metal impurities or the like arecaused thereby.

Furthermore, when heat treatment is conducted in a hydrogen atmosphere,it is necessary to take out the wafer after replacing the atmosphere inthe heat treatment furnace with nitrogen gas for safety. At that time,the surface of the wafer is locally etched with slight amount of oxygenand water vapor contained in nitrogen gas, which may degrading surfaceroughness such as haze or the like, which may lead to bonding failurewhen they are bonded.

Recently, it has been reported that there can be produced CZ waferwherein Grown-in defects are significantly reduced if crystal is pulledwith strictly controlling a growth rate and temperature gradient ofsolid-liquid interface while single crystal is grown according toCzochralski method. It can be easily expected that SOI wafer having fewdefects in SOI layer can be produced if such a wafer is used as a bondwafer. However, if the crystal is pulled under such significantly strictgrowing condition may naturally lead to lowering in yield, resulting insignificant increase of cost for production.

On the other hand, single crystal produced according to FZ method has noCOP defects as observed in CZ single crystal, but FZ crystal having adiameter more than 150 mm cannot be produced at commercial level.Although FZ crystal having a diameter of 200 mm can be produced atexperimental level, there is no hope for producing a large diameterwafer having a diameter of 300 mm, 400 mm in the future.

DISCLOSURE OF THE INVENTION

The present invention has been accomplished to solve the above-mentionedproblems. A main object of the present invention is to provide a SOIwafer that has a SOI layer having few crystal defects and high qualityin high productivity, in high yield and with low cost by using a waferwherein grown-in defects in a surface-layer part of silicon singlecrystal wafer produced by CZ method are eliminated or reducedeffectively by heat treatment as a bond wafer of a bonded wafer.

To achieve the above mentioned object, the present invention provides amethod of producing a bonded SOI wafer comprising bonding a bond waferand a base wafer via an oxide film and then reducing thickness of thebond wafer, wherein a silicon single crystal ingot is grown according toCzochralski method, the single crystal ingot is then sliced to produce asilicon single crystal wafer, the silicon single crystal wafer issubjected to heat treatment in a non-oxidizing atmosphere at atemperature of 1100° C. to 1300° C. for one minute or more andcontinuously to a heat treatment in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to provide asilicon single crystal wafer wherein a silicon oxide film is formed onthe surface, and the resultant wafer is used as the bond wafer.

As described above, if the wafer produced according to Czochralskimethod is subjected to heat treatment in a non-oxidizing atmosphere at atemperature of 1100° C. to 1300° C. for one minute or more andcontinuously to a heat treatment in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to provide asilicon single crystal wafer wherein a silicon oxide film is formed onthe surface, and the resultant wafer is used as the bond wafer, asilicon single crystal wafer having high quality wherein Grown-indefects near the surface of the wafer that are harmful for fabricationof semiconductor device can be eliminated or decreased in short time canbe used as a bond wafer, so that SOI wafer that has a SOI layer havingfew crystal defects and high quality can be produced in highproductivity, in high yield with low cost.

The present invention also provides a method of producing a bonded SOIwafer comprising bonding a bond wafer and a base wafer via an oxide filmand then reducing thickness of the bond wafer, wherein a silicon singlecrystal ingot is grown according to Czochralski method, the singlecrystal ingot is then sliced to produce a silicon single crystal wafer,the silicon single crystal wafer is subjected to heat treatment in anon-oxidizing atmosphere at a temperature of 1100° C. to 1300° C. forone minute or more and continuously to a heat treatment in an oxidizingatmosphere at a temperature of 700° C. to 1300° C. for one minute ormore without cooling the wafer to a temperature less than 700° C. toprovide a silicon single crystal wafer wherein a silicon oxide film isformed on the surface, at least one of hydrogen ions and rare gas ionsare implanted into the surface via a silicon oxide film of the wafer toform an ion implanted layer, and the resultant wafer is used as the bondwafer, which is then brought into close contact with the base wafer viathe silicon oxide film of the bond wafer, followed by delamination atthe ion implanted layer by heat treatment.

As described above, in method of producing a bonded SOI wafer, by usingthe method wherein the wafer produced according to Czochralski method issubjected to heat treatment in a non-oxidizing atmosphere at atemperature of 1100° C. to 1300° C. for one minute or more andcontinuously to a heat treatment in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to provide asilicon single crystal wafer wherein a silicon oxide film is formed onthe surface, at least one of hydrogen ions and rare gas ions areimplanted into the surface via a silicon oxide film of the wafer to forman ion implanted layer, and the resultant wafer is used as the bondwafer, which is then brought into close contact with the base wafer viathe silicon oxide film of the bond wafer, followed by delamination atthe ion implanted layer by heat treatment (so called ion implantationdelamination method), a silicon single crystal wafer having high qualitycan be used as a bond wafer, and surface condition of the SOI waferafter delamination is good, so that SOI wafer having excellent thicknessuniformity can be produced by a relatively easy method.

In that case, the bond wafer delaminated at the ion implanted layer inthe above-mentioned method of producing a bonded SOI wafer of thepresent invention can be used as a new bond wafer.

As described above, in the bond wafer delaminated at the ion implantedlayer in the method of producing a bonded SOI wafer of the presentinvention, grown-in defects in zone at a depth of about 5 to 10 μm ormore from the surface are eliminated, and thickness of the thin filmdelaminated at the ion implanted layer is about one μm at thickest, sothat the bond wafer has denuded (low-defect) zone with sufficient depth,even though it is a wafer after delamination of a thin film.Accordingly, even if the surface of the wafer is polished for reuse,sufficient denuded zone remains. Therefore, if it is used as a new bondwafer, and bonded to the base wafer via the oxide film, and thickness ofthe bond wafer is decreased to produce a SOI wafer, it is not necessaryto conduct further heat treatment of the bond wafer before bonding forelimination of grown-in defects. Thereby, a bonded SOI wafer having highquality can be produced efficiently.

Furthermore, the bond wafer delaminated at the ion implanted layer inthe above-mentioned method of producing a bonded SOI wafer of thepresent invention can be used as a new base wafer.

At an inner part (a bulk part) than denuded zone near the surface of thebond wafer after delamination of a thin film, a lot of oxideprecipitates are sometimes generated due to influence of heat treatment.In that case, if the wafer is used as a new base wafer, and bonded tothe bond wafer via the oxide film, and thickness of the bond wafer isdecreased to produce a SOI wafer, the SOI wafer having high performancein gettering of heavy metal impurities or the like can be obtained. Inthat case, even though a lot of oxide precipitates are generated in abulk part, a surface-layer part is denuded zone as described above, sothat oxide precipitates are never exposed on the surface of the basewafer, and there is no adverse effect to bonding with a bond wafer.

The above-mentioned non-oxidizing atmosphere is preferably argon,nitrogen or a mixed gas of argon and nitrogen.

Because, the atmosphere of argon, nitrogen or a mixed gas of argon andnitrogen can be easily handled and inexpensive.

The above-mentioned oxidizing atmosphere may contain water vapor.

As described above, if the oxidizing atmosphere contains water vapor, anoxidation rate is high, and defects can be eliminated efficiently inquite short time by injection of interstitial silicon. Since the oxidefilm formed on the bond wafer gets relatively thick, it is suitable forproduction of SOI wafer having a thick BOX.

In that case, the above-mentioned oxidizing atmosphere can be dry oxygenatmosphere or a mixed gas atmosphere of dry oxygen and argon ornitrogen.

As described above, if the oxidizing atmosphere is dry oxygen atmosphereor a mixed gas atmosphere of dry oxygen and argon or nitrogen, a growthrate of the oxide film is low, and thickness of the oxide film formed onthe surface of the bond wafer after heat treatment can be made thin, andthus, it is suitable for production of the SOI wafer having thin BOX.

The thickness of the oxide film formed by the above-mentioned heattreatment in the oxidizing atmosphere is preferably 20 to 100 nm.

As described above, if the thickness of the oxide film formed by theabove-mentioned heat treatment in the oxidizing atmosphere is more than20 nm, COP at a surface-layer part of the bond wafer can be removedsufficiently. If the thickness is 100 nm or less, time necessary for thestep can be short even in the case that the formed oxide film needs tobe removed. Furthermore, in the case that the SOI wafer is producedaccording to the above-mentioned ion implantation delamination method,thickness uniformity of the SOI layer gets better, since an absolutevalue of the deviation in thickness of the oxide film on the surfacegets small.

The oxide film can be previously formed on the surface of the waferbefore the heat treatment in a non-oxidizing atmosphere.

If such an oxide film is previously formed, the surface of the wafer canbe protected so that formation of thermal nitride film on the surface ofthe wafer due to heat treatment or surface roughness due to etching canbe prevented. Therefore, bonding failure when the wafers are bonded canbe prevented.

In that case, thickness of the thermal oxide film on the surface of thewafer after the above-mentioned heat treatment in the oxidizingatmosphere is preferably 300 nm or more.

As described above, if the thermal oxide film having a thickness of 300nm or more is grown, COP on the surface of the wafer can be eliminatedby reflow phenomenon of silicon oxide during growth of the oxide filmeven when the oxide film is previously formed on the surface of thewafer before conducting the heat treatment in the non-oxidizingatmosphere, so that COP on the surface of the wafer can be eliminatedmore surely.

A silicon single crystal ingot is preferably grown according toCzochralski method with controlling a cooling rate at 1150° C. to 1080°C. of the single crystal ingot to be 2.3° C./min or more.

As described above, if a silicon single crystal ingot is grown accordingto Czochralski method with controlling a cooling rate at 1150° C. to1080° C. of the single crystal ingot to be 2.3° C./min or more, a sizeof grown-in defect gets small. Since the above-mentioned heat treatmentis conducted to such a wafer, grown-in defects in a surface-layer partof the wafer can be eliminated or reduced more efficiently. Accordingly,a SOI wafer having a SOI layer with higher quality can be produced inhigh productivity.

In that case, it is preferable that a silicon single crystal ingot inwhich nitrogen is doped is grown according to Czochralski method.

As described above, if a silicon single crystal ingot in which nitrogenis doped is grown according to Czochralski method, the size of grown-indefect becomes smaller by nitrogen doping. Further, and the heattreatment is conducted thereto, and thus grown-in defects in asurface-layer part of the wafer can be more efficiently eliminated orremoved. Accordingly, a SOI wafer having a SOI layer with higher qualitycan be obtained in high productivity.

In that case, when growing silicon single crystal ingot in whichnitrogen is doped according to Czochralski method, the concentration ofnitrogen doped in the single crystal ingot is preferably 1×10¹⁰ to5×10¹⁵ atoms/cm³.

Because, 1×10¹⁰ atoms/cm³ or more is preferable in order to suppressgrowth of grown-in defects sufficiently, and 5×10¹⁵ atoms/cm³ or less ispreferable in order not to prevent formation of single crystal ofsilicon single crystal.

Furthermore, when the silicon single crystal ingot is grown according toCzochralski method, the concentration of oxygen contained in the singlecrystal ingot is preferably 18 ppma (JEIDA: Japan Electronic IndustryDevelopment Association) or less.

If oxygen concentration is low as described above, growth of crystaldefects can be suppressed further, and formation of oxide precipitatesat a surface layer can also be prevented.

A bonded SOI wafer produced according to the method of the presentinvention is, for example, a bonded SOI wafer wherein a SOI layerconsists of CZ silicon single crystal wafer, thickness of the SOI layeris 5 μm or less, and 1.3 number/cm² or less of COP having a size of 0.09μm or more exist at any region in depth direction of the SOI layer.

As described above, in the bonded SOI wafer of the present invention,there exist very few COP at any region in depth direction of SOI layer,even if thickness of SOI layer is more than 0.5 μm. Furthermore, SOIwafer of the present invention does not need to be subjected to hydrogenanneal or the like after production of SOI wafer, and thus productivityis also high.

According to the present invention, it is possible to eliminate voiddefects in deeper region efficiently compared with conventional methods,and therefore SOI layer with high quality can be formed. Furthermore,since heat treatment in non-oxidizing atmosphere and heat treatment inoxidizing atmosphere can be conducted in the same batch, the number ofthe steps in production of SOI does not increase, and thus cost therefordoes not increase either. Furthermore, since heat treatment can beconducted without using hydrogen, heat treatment can be conducted withno danger of contamination from the furnace due to hydrogen andexplosion. Furthermore, since CZ wafer is used, it can be applied to awafer having a large diameter as 300 mm or more.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing one example of production processes of abonded SOI wafer according to the present invention.

FIG. 2 is a view showing outline of heat treatment applied to a siliconsingle crystal wafer to be a bond wafer before bonding.

FIG. 3 is a graph showing number of COP in the wafer after heattreatment in Example 1, Comparative Example 1 and Comparative Example 2.

FIG. 4 is a graph showing TZDB good chip yield in the wafer after heattreatment in Example 1, Comparative Example 1 and Comparative Example 2.

FIG. 5 is a graph showing TDDB good chip yield in the wafer after heattreatment in Example 1, Comparative Example 1 and Comparative Example 2.

FIG. 6 is a graph showing number of COP on the surface of the waferafter heat treatment in Examples 1 to 3.

FIG. 7 is a graph showing TZDB good chip yield in the SOI wafer ofExample 4.

FIG. 8 is a graph showing TDDB good chip yield in the SOI wafer ofExample 4.

FIG. 9(a) is a graph showing a relation between oxygen concentration inannealing atmosphere and number of COP, a FIG. 9(b) is a graph showing arelation between a thickness of oxide film formed by annealing andnumber of COP.

FIG. 10 is a graph showing comparison of transition of contaminationlevel of metal impurities when each of heat treatment of the presentinvention and conventional invention is conducted in different tubesrepeatedly.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiment of the present invention will be further described below,but is not limited thereto.

The inventors have found that grown-in defects on the surface and in asurface-layer part of the wafer can be eliminated or reduced, andsurface roughness of the wafer can be improved by producing a siliconwafer according to Czochralski method and then subjecting the wafercontinuously to heat treatment at high temperature in non-oxidizingatmosphere, especially in atmosphere of argon, nitrogen, or mixedatmosphere thereof, and oxidation heat treatment at high temperature inoxidizing atmosphere, and that a SOI wafer having an excellent SOI layercan be produced in high productivity by using the silicon wafer as abond wafer of a bonded SOI wafer, and have studied condition further,and thereby completed the present invention.

As described above, in order to eliminate or reduce, grown-in defects onthe surface and in a surface-layer part of the wafer, a wafer usedgenerally in commercial level is a wafer grown at a general crystalgrowth rate of about 1.0 mm/min or more and subjected to heat treatmentat high temperature in hydrogen atmosphere to eliminate grown-indefects. The method has already been used for actual production ofdevices, but defects still remains in a surface-layer part of the wafer(for example, 0 to 5 μm from the surface).

Reasons therefor have been considered as follows. Two steps arenecessary in order to eliminate grown-in defects that are aggregationsof atomic vacancy. Namely, they are a step of melting an inner walloxide film of defects, which prevents true point defects from changinginto grown-in defects, and a subsequent step of filling the grown-indefect with interstitial silicon.

In the heat treatment at high temperature in hydrogen atmosphere, it isconsidered that melting of an inner wall oxide film of grown-in defectsin a surface-layer part of the wafer can be efficiently caused bysignificant oxygen out-diffusing effect. However, filling of grown-indefect with interstitial silicon cannot be caused efficiently, sinceboth of interstitial silicon that is a Schottky defect and atomicvacancy are injected from the surface of the wafer under heat treatmentat high temperature in hydrogen atmosphere.

Accordingly, it takes long time for a step of filling grown-in defectswith interstitial silicon in heat treatment at high temperature inhydrogen atmosphere. Especially, in order to eliminate grown-in defecthaving a size of 150 nm or more in terms of their diameter, it isnecessary to perform a heat treatment at high temperature as 1200° C.for long time as 5 hours or more.

It lowers productivity of the wafer, and is not preferable from a pointof safety, since it requires heat treatment at high temperature inhydrogen atmosphere for long time. Furthermore, since heat treatment athigh temperature for long time is conducted, oxygen precipitation nucleiin silicon single crystal wafer are also eliminated, gettering effect ofheavy metal that is effective for device process are also lost.

The inventors solved the above problems by conducting heat treatment ina non-oxidizing gas containing hydrogen less than explosion limit (about4%), especially atmosphere of argon, nitrogen or mixture thereof, at atemperature of 1100° C. to 1300° C. for one minute or more andcontinuously to a heat treatment in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. Namely, a step ofmelting of an inner wall oxide film of defects is efficiently conductedby heat treatment at high temperature in atmosphere of argon, nitrogenor a mixed gas thereof, and then heat treatment in oxidizing atmosphereis continuously conducted to progress efficiently a step of fillinggrown-in defect with interstitial silicon.

The step of melting of an inner wall oxide film of defects isefficiently conducted by heat treatment at high temperature inatmosphere of non-oxidizing gas containing hydrogen less than explosionlimit (about 4%), especially, argon, nitrogen or a mixed gas thereof,since it is quite difficult from the point of safety that heat treatmentin oxygen atmosphere is continuously conducted after heat treatment athigh temperature in hydrogen atmosphere. Two steps of heat treatment canbe continuously safely conducted only by using atmosphere ofnon-oxidizing gas containing hydrogen less than explosion limit (about4%) especially, argon, nitrogen or a mixed gas thereof, instead ofhydrogen atmosphere. Moreover, due to oxygen out-diffusion effect,melting of inner wall oxide film of grown-in defects is conductedefficiently in heat treatment at high temperature in atmosphere ofnon-oxidizing gas, especially argon, nitrogen or mixed gas thereof, aswell as it is conducted in hydrogen atmosphere.

It is considered that oxygen out-diffusion effect in argon atmosphere isequivalent to that in hydrogen atmosphere, since natural oxide film onthe surface of the wafer is sublimated as SiO gas and removed in heattreatment at high temperature as 1100 to 1300° C. in argon atmosphere.In nitrogen atmosphere, out-diffusion effect is equivalent to that inhydrogen atmosphere, but natural oxide film on the surface cannot beremoved uniformly. Accordingly, it is preferable to remove the naturaloxide film previously before heat treatment with HF aqueous solution.Furthermore, in a mixed atmosphere of argon and nitrogen, out-diffusioneffect is equivalent to that achieved in hydrogen atmosphere.

The heat treatment is conducted at 1100 to 1300° C. for one minute ormore in order to melt sufficiently inner wall oxide film of the grown-indefect.

In heat treatment at high temperature in nitrogen atmosphere, verystable thermal nitride film is formed on the surface of the siliconwafer, and it sometimes takes long time to remove the film in the laterprocesses, or surface roughness on the surface of the wafer is sometimescaused by slight amount of oxygen or water vapor in nitrogen. Theinventors have found that formation of unnecessary film and surfaceroughness on the surface of the wafer caused in nitrogen atmosphere canbe prevented by previously forming a protection oxide film on thesurface of the wafer before heat treatment.

Furthermore, with such a protection oxide film, in addition to theeffect of protection of the surface of the wafer against formation ofunnecessary film and surface roughness, an effect of preventingcontamination with heavy metal impurities diffused inside of the waferfrom the furnace during heat treatment can be achieved.

Furthermore, a step of melting the inner wall oxide film of defect and astep of filling grown-in defects with interstitial silicon arecontinuously conducted from the following reason. If these two steps ofheat treatment are not conducted continuously, the inner wall oxide filmof grown-in defect is grown again due to lowering of temperature of thewafer, and as a result defects cannot be eliminated or reduced.Therefore, these two steps of heat treatment are continuously conductedwithout cooling to the temperature less than 700° C. at which the innerwall oxide film of grown-in defects begin to grow again.

In the present invention, a step of filling grown-in defect is conductedby heat treatment in oxidizing atmosphere. Because, according to theheat treatment in oxidizing atmosphere, differently from the heattreatment in hydrogen atmosphere, atomic vacancy is never injected, butonly interstitial silicon is injected from the surface of the wafer, sothat grown-in defects can be efficiently filled with interstitialsilicon, and grown-in defects can be eliminated, and surface roughnessand contamination can be prevented by oxidizing the surface activated bythe heat treatment at high temperature in non-oxidizing atmosphere.

In order to fill and eliminate grown-in defects sufficiently, the heattreatment is preferably conducted at 1000° C. to 1300° C. for one minuteor more. However, at temperature of 700° C. or more, reduction ofgrown-in defects and prevention of surface roughness can be achieved.

As the oxidizing atmosphere, atmosphere containing water vapor, dryoxygen (dry O₂) 100% atmosphere, or a mixed gas of dry oxygen and argonor nitrogen atmosphere or the like can be adopted.

Using atmosphere containing water vapor, an oxidation rate is high, sothat interstitial silicon can be injected efficiently in very short timeto eliminate defects at a relatively low temperature around 700° C. Inthat case, thickness of the oxide film formed on the surface isrelatively thick, it is suitable for the case that SOI having a thickburied oxide film is produced.

On the other hand, using dry oxygen atmosphere or a mixed gas atmosphereof dry oxygen and argon or nitrogen, a growing rate of an oxide film islow, thickness of the oxide film formed after heat treatment can bethin. Therefore, it is suitable for the case that the formed oxide filmneeds to be removed with an aqueous solution of HF or the like, or thecase that the above-mentioned ion implantation delamination method isused.

If a growth rate of the oxide film is low and thickness of the formedoxide film is thin, in the case that the mixed gas atmosphere is used orthe like, it has been considered that the effect of eliminating defectsby injecting the interstitial silicon is inferior. The inventorsperformed the following experiments to confirm what oxygen concentrationand thickness of the oxide film are enough for sufficiently eliminatingdefects.

Annealing in argon 100% atmosphere, at 1200° C., for 40 minutes wasconducted, and then annealing in 6 kinds of mixed gas of argon and dryoxygen each of which has different oxygen concentration (oxygenconcentration 0, 10, 20, 30, 50, 100%) for 20 minutes was conducted.Then, the surface of the wafer was polished with a stock removal of 5μm, and COP having a size of 0.9 μm or more was measured. The resultswere shown in FIGS. 9(a), (b). Polishing with a stock removal of 5 μmwas conducted in order to observe effect of elimination of COP in asurface-layer part of the wafer. FIG. 9(a) shows a relation betweenoxygen concentration in annealing atmosphere and the number of COP. FIG.9(b) shows a relation between thickness of the oxide film formed byannealing and the number of COP.

As shown in FIG. 9, even if dry oxygen concentration in a mixed gasatmosphere is only about 10%, in the case that the thickness of theoxide film is 20 nm or more, effect equivalent to that in the case ofusing dry oxygen 100% (thickness of oxide film is about 100 nm) can beobtained.

Furthermore, it has been found that heat treatment in oxidizingatmosphere after the heat treatment in non-oxidizing atmosphere has aneffect of preventing contamination of the wafer from a tube or a boat toa minimum. FIG. 10 shows comparison of change of contamination level ofmetal impurities in the wafer in the case that annealing in argon 100%atmosphere at 1200° C. for 40 minutes is conducted and then heattreatment in an atmosphere of mixed gas of argon and dry oxygen (oxygenconcentration 30%) for 20 minutes is conducted, and in the case thatheat treatment in hydrogen 100% or argon 100% atmosphere at 1200° C. for60 minutes is conducted, in each case different tube is used, and heattreatment was conducted repeatedly. Measurement of contamination levelwas conducted using SPV (Surface Photo Voltage: trade name, a wafercontamination monitoring system) manufactured by SemiconductorDiagnostic Inc. (SDI).

It is clear that annealing only with hydrogen or argon may cause etchingof a tube or a boat, which may lead to sudden degrading of impuritylevel. On the other hand, as for the heat treatment containing annealingin oxidizing atmosphere, oxide film is formed on the surface of thewafer and the surface of a boat or a tube, and therefore the oxide filmfor protection are always formed, and thus it is considered thatcontamination from a tube or a boat can be suppressed to a minimum.

Defects that can be eliminated by the oxidation heat treatment at 700 to1300° C. are limited to crystal defects that are not exposed on thesurface of the silicon wafer and exist inside of the wafer. Because,elimination of the defects herein is conducted by injecting interstitialsilicon from the surface due to oxidation to fill the void type crystaldefects therewith. Accordingly, the void type crystal defects such asCOP that are exposed on the surface need to be previously eliminated bymigration of silicon atoms on the surface of the wafer by heat treatmentin argon atmosphere or the like before the oxidation heat treatment.However, if surface protection oxide film is formed, migration of thesurface silicon atoms is suppressed, so that COP on the surface may beeliminated insufficiently.

The inventors thought out a method of eliminating COP on the surface ofthe wafer sufficiently by forming a thermal oxide film with a thicknessof 300 nm or more on the surface of the wafer after the above-mentionedoxidation heat treatment at 700 to 1300° C., in the case of previouslyforming surface protection oxide film before heat treatment innon-oxidizing atmosphere. Because, if thickness of the thermal oxidefilm on the surface of the wafer after oxidation heat treatment is 300nm or more, the shape of COP on the surface gets smooth during a processof growing the thermal oxide film, and substantially the same effect aselimination of COP can be achieved. An average size of COP on thesurface of the wafer is 100 to 200 nm. If an oxide film having athickness about 300 nm is formed, it is enough to take COP into theoxide film and eliminate them.

The oxide film formed by the oxidation heat treatment can be removedwith an aqueous solution of HF or the like.

Furthermore, the inventors have found that silicon wafer having a largesize and have few grown-in defects can be produced in high productivityby a method of growing silicon single crystal ingot according toCzochralski method with controlling a cooling rate at 1150° C. to 1080°C. to be 2.3° C./min or more, and a method of growing silicon singlecrystal ingot in which nitrogen is doped, and that the effect ofeliminating or reducing grown-in defects can be improved by subjectingthe said silicon wafer to the above-mentioned non-oxidizing heatingtreatment and oxidation heat treatment of the present invention.

Namely, it is said that grown-in defects aggregates at temperature inthe range of 1150° C. to 1080° C. during pulling of crystal.Accordingly, if a cooling rate in the temperature range of 1150 to 1080°C. is made 2.3° C./min or more and staying time is shorten, size andnumber of grown-in defect can be controlled.

It is pointed out that the agglomeration of atomic vacancies in siliconsingle crystal can be suppressed, if nitrogen is doped in the siliconsingle crystal (T. Abe and H. Takeno, Mat. Res. Soc. Symp. Proc.Vol.262,3,1992). It is considered that the effect can be achieved as aresult that vacancy agglomeration process is transited from homogenousnucleus formation to heterogeneous nucleus formation. Accordingly,silicon single crystal having small grown-in defect can be obtained bygrowing the silicon single crystal by CZ method with doping nitrogen,and thus the silicon single crystal wafer can be obtained by processingit. According to the method, it is not always necessary to decreasegrowth rate of the crystal, differently from the conventional method,and thus a silicon single crystal wafer can be produced in highproductivity.

It is preferable that oxygen concentration in a single crystal ingot is18 ppma or less, when the silicon single crystal ingot is grown byCzochralski method. Because, if oxygen concentration is low as above,growth of crystal defects can be further suppressed, and formation ofoxide precipitates near the surface of the wafer can be prevented.Especially, when nitrogen is doped in single crystal, oxygenprecipitation is accelerated, and thus it is preferable that formationof oxide precipitates near the surface of the wafer is prevented byusing the oxygen concentration in the above-mentioned range.

In the present invention, control of size and number of grown-in defectswith a cooling rate in Czochralski method can be performed by changing apulling rate of the crystal. For example, when a certain specificpulling apparatus is used, a cooling rate achieved with a pulling rateof 1.8 mm/min is higher than a cooling rate achieved with a pulling rateof 1.0 mm/min by the same apparatus. As other methods, position andstructure or the like of members in the furnace of a pulling apparatuscalled hot zone can be changed to control a cooling rate at atemperature of 1150-1080° C.

The size of grown-in defect can also be controlled by doping nitrogenimpurity while single crystal is grown according to Czochralski method.In that case, silicon single crystal ingot in which nitrogen is dopedcan be grown by a known method such as disclosed in, for example,Japanese Patent Application Laid-open (kokai) No 60-251190.

Namely, nitrogen can be doped in a silicon single crystal by placingnitride previously in the quartz crucible before growing silicon singlecrystal ingot, adding nitride into the silicon melt, or by using anatmosphere gas containing nitrogen. The doping amount in the crystal canbe controlled by controlling the amount of nitride, concentration ortime of introduction of nitrogen gas.

As described above, agglomeration of grown-in defects can be suppressedby doping nitrogen when the single crystal is grown according toCzochralski method.

As for the reason for the size reduction of crystal defects introducedinto silicon when nitrogen is doped in the silicon single crystal,atomic vacancy agglomeration process is transited from homogenousnucleus formation to heterogeneous nucleus formation as described above.

Accordingly, the concentration of nitrogen to be doped is preferably1×10¹⁰ atoms/cm³ or more, more preferably 5×10¹³ atoms/cm³ or more, inwhich ranges the heterogeneous nucleus formation is sufficiently caused.Thereby, agglomeration of crystal defects can be sufficientlysuppressed.

On the other hand, if nitrogen concentration is more than 5×10¹⁵atoms/cm³, which is solid solubility of nitrogen in silicon singlecrystal, crystallization of the silicon single crystal is inhibited.Therefore, it should not be more than the above concentration.

In the present invention, it is preferable that oxygen concentration inthe single crystal ingot is 18 ppma or less, when the silicon singlecrystal ingot is grown by Czochralski method. Oxygen concentrationcontained in the single crystal ingot can be lowered so as to fall inthe above range by a conventional method, when a silicon single crystalingot is grown. For example, oxygen concentration can be easilycontrolled to fall in the above mentioned range by reducing the numberof rotation of a crucible, increasing volume of flowing gas, lowering anatmosphere pressure, controlling temperature distribution and convectionof a silicon melt or the like.

Thereby, the silicon single crystal ingot wherein size and number ofgrown-in defects are reduced, can be thus obtained according toCzochralski method. After it is sliced according to a general methodwith a cutting machine such as an inner diameter slicer, a wire saw orthe like, it is subjected to processes including chamfering, lapping,etching, polishing and the like to be a silicon single crystal wafer. Ofcourse, such processes are merely examples, and various other processessuch as cleaning or the like can be conducted, and process can bechanged appropriately depending on the purpose, namely, order ofprocesses can be changed, and some processes can be omitted.

Thereby, CZ silicon single crystal wafer used as a bond wafer in thepresent invention can be obtained. A method of producing SOI wafer ofthe present invention using the CZ silicon wafer will be explainedbelow. FIGS. 1(A) to (E) is a flow chart showing one example of theprocess for producing a bonded SOI wafer of the present invention. FIG.2 shows outline of the heat treatment to which the silicon singlecrystal wafer to be a bond wafer is subjected before bonding.

First, CZ silicon single crystal wafer 5 that is to be a bond wafer issubjected to heat treatment consisting of two steps shown in FIGS. 1(B)and (C) and FIG. 2. First, as the first step, annealing is conducted ina temperature range of 1100° C. to 1300° C. in 100% Ar gas atmospherefor more than one minute to out-diffuse oxygen in the crystal and meltoxide film inner wall of void defects. Thereby, low-defect layer 3 isformed in the silicon single crystal wafer 5 (FIG. 1(B), FIG. 2). Then,annealing is conducted continuously in an oxidizing atmosphere at atemperature of 700° C. to 1300° C. for one minute or more withoutcooling the wafer to a temperature less than 700° C. to form an oxidefilm 4, to inject interstitial silicon from an interface of Si/SiO₂, andeliminate void defects to a deeper part of the crystal, and enlarge thelow-defect layer 3 (FIG. 1(C), FIG. 2). According to the method, COP canbe effectively eliminated from the surface to the depth of about 5 to 10μm or more.

The heat treatment can be conducted using any types of heat treatmentfurnace commercially available, as far as it is a heat treatment furnacewherein cleanness is controlled.

For example, a heater heating horizontal type or vertical type diffusionfurnace can be used, or a lamp heating type single wafer processingwafer heating apparatus can be used. It is important that sufficientheat treatment temperature and heat treatment time in non-oxidizingatmosphere and heat treatment temperature and heat treatment time forthe subsequent heat treatment in oxidizing atmosphere are ensured inorder to eliminate or reduce grown-in defects effectively, and that twoheat treatments are conducted continuously so that temperature is notlowered too much.

Therefore, it is necessary to subject silicon single crystal wafer 5 toheat treatment in a non-oxidizing atmosphere, especially argon, nitrogenor mixed gas of argon and nitrogen at a temperature of 1100 to 1300° C.for one minute or more, and then to subsequent oxidation in an oxidizingatmosphere at a temperature of 700° C. to 1300° C. for one minute ormore without cooling the wafer to a temperature less than 700° C.

As described above, if the heat treatment in a non-oxidizing atmosphere,especially argon, nitrogen or mixed gas of argon and nitrogen and thesubsequent oxidation heat treatment are not conducted continuously, theinner wall oxide film of grown-in defects are grown, and as a result thedefects cannot be eliminated or reduced. Accordingly, it is preferableto conduct continuously the heat treatment in an atmosphere of argon orthe like, and subsequent oxidation heat treatment, before the wafer 5 iscooled to the temperature less than 700° C., without taking out thewafer 5 from the furnace. Furthermore, heat treatment time can beshorten, since the heat treatments are conducted continuously at thesame temperature.

In order to conduct the heat treatment as above, after heat treatment inan atmosphere of argon, nitrogen or a mixed gas of argon and nitrogen orthe like is conducted, the atmosphere gas was exhausted, and oxygen gasis subsequently introduced at desired concentration to conduct theoxidation heat treatment. According to the present invention, since theinitial heat treatment for melting an inner wall oxide film of defectsis conducted in non-oxidizing gas atmosphere such as argon, nitrogen ora mixed gas of argon that contains hydrogen in an amount less thanexplosion limit (about 4%), the next oxidation heat treatment can besafely even if a conventional commercially available furnace is used.

Furthermore, in the case that a protection oxide film is previouslyformed on the surface of the wafer that is to be subjected to the heattreatment for melting of inner wall oxide film of defects innon-oxidizing atmosphere such as argon, nitrogen or a mixed gas of argonand nitrogen, the heat treatment for forming the above oxide film can beconducted continuously before the heat treatment for melting an innerwall oxide film, or it can be conducted previously as an independentstep. The oxide film may be formed, for example, by thermal oxidationsuch as so-called dry oxidation using dry oxygen, wet oxidationcontaining water vapor, or CVD oxide film formed by CVD (Chemical VaporDeposition) method.

Heat treatment in oxidizing atmosphere as the second step shown in FIG.1(C) and FIG. 2 can be conducted by either dry oxidation in anatmosphere containing no water vapor or wet oxidation in an atmospherecontaining water vapor. In both the ways above, equivalent effect can beexpected as for injection of interstitial silicon into grown-in defectsand improvement in surface roughness that are essential objects of thepresent invention.

Then, as shown in FIG. 1(D), a bonded SOI wafer is produced using theabove silicon single crystal wafer on which the silicon oxide film isformed as a bond wafer 1. As shown in FIG. 1(D), since the oxide film 4formed in the preceding step is used as BOX of SOI wafer, steps can besimplified. Furthermore, since an oxide film is formed afterAr-annealing, BOX can be formed excellent in film quality. The wafer isbrought in close contact with a base wafer 2 via the BOX at roomtemperature, and subjected to the bonding heat treatment at 200° C. ormore, generally at about 1000° C. to 1200° C. As the base wafer 2, asilicon single crystal wafer is generally used, but insulator substrate(quartz, sapphire or the like) can also be used depending on the use.Furthermore, in the case that a silicon single crystal wafer is used, itcan be bonded to a base wafer 2 after an oxide film is formed on thebase wafer.

After conducting the bonding heat treatment, SOI wafer 10 is produced byconducting general process for decreasing thickness such as grinding,polishing or the like (FIG. 1(E)). Thereby, there can be obtained thebonded SOI wafer 10 wherein BOX 12 consisting of the oxide film 4 andthe SOI layer 11 consisting of the low-defect layer 3 are formed on thebase wafer 2. Since the SOI layer 11 of the SOI wafer 10 consists of thelow-defect layer 3, there are very few defects such as COP at any regionin depth direction. In that case, a vapor phase etching called PACE(Plasma Assisted Chemical Etching) can be conducted to decreasethickness of the bond wafer 1 (Japanese Patent No. 256567).

In the case that SOI wafer is produced using ion implantationdelamination method (technique called smart-cut, Japanese PatentApplication Laid Open (Kokai) No. 5-211128), an oxide film 4 is formedon the surface of the silicon single crystal wafer 5 by theabove-mentioned two step heat treatment, and then hydrogen ions or raregas ions are implanted through the oxide film 4, and it is used as abond wafer 1, which is then bonded to the base wafer 2.

In that case, since deviation in thickness of the formed SOI layer is atotal of deviation in depth of ion implantation and deviation ofthickness of the oxide film, in order to decrease it as possible, it isdesirable to decrease thickness of the oxide film 4 formed on thesilicon single crystal wafer 5 that is to be a bond wafer 1 as possibleto decrease an absolute value of deviation in thickness of the oxidefilm prepared. Therefore, thickness of the oxide film is preferably 100nm or less, and preferably 20 nm or more in order to obtain effect ofeliminating defects.

In the case that thickness of the oxide film formed on the bond wafer is100 nm or less as described above, and a thicker BOX of SOI wafer isnecessary from the point of device design, it may be bonded to the basewafer after the lacking oxide film is formed.

Furthermore, the bond wafer that is delaminated when SOI wafer isproduced according to such a hydrogen ion implantation delaminationmethod can be used as a new bond wafer or a base wafer. As describedabove, the bond wafer produced as a byproduct after delamination has adenuded zone with sufficient depth in a surface-layer part, and containsa sufficient amount of oxide precipitate precipitated in a bulk part dueto heat treatment, so that it can be a good bond wafer or a good basewafer.

In that case, one surface of the wafer produced as byproduct in thepresent invention is a delaminated surface, and the surface of the otherside is a plain surface of original silicon wafer. Accordingly,treatment such as grinding, polishing or the like has to be applied onlyon the delaminated surface. Accordingly, the process is simple sinceonly one surface is treated, and stock removal is slight. Namely, when asilicon wafer is provided by slicing a general silicon ingot, bothsurfaces are cut surface, so that steps such as a lapping step, anetching step or the like are necessary, and the stock removal is large.However, the delaminated wafer of the present invention has a plainsurface on one side, and therefore only the delaminated surface has tobe ground or polished on the basis of the plain surface, and the sameplain surface as a general silicon mirror surface wafer can be obtainedwith a slight stock removal.

If the silicon wafer obtained by reprocessing the delaminated wafer isreused as a bond wafer or a base wafer of a SOI wafer, substantially oneSOI wafer can be obtained from one silicon wafer, so that silicon wafercan be used effectively as material.

EXAMPLES

The following examples and comparative examples are being submitted tofurther explain the present invention. These examples are not intendedto limit the scope of the present invention.

Example 1, Comparative Example 1, Comparative Example 2

In accordance with the method of the present invention, a bond wafer ofa bonded SOI wafer was produced, and quality thereof was evaluated. As asilicon single crystal wafer that is to be a bond wafer, a wafer slicedfrom 8″φCZ silicon single crystal having orientation <100>, interstitialoxygen concentration of 16 ppma (JEIDA) pulled with a pulling rate of1.2 mm/min was used.

The wafer was subjected to the heat treatment of the present invention.Annealing was conducted at 1200° C. for 40 minutes in an atmosphere of100% Ar using VERTEX3(DD-813V) manufactured by Kokusai Electric Co.,Ltd., as an annealing furnace, and subsequently annealing at the sametemperature in an atmosphere of a mixed gas of 30% oxygen and 70% argonfor 20 minutes was conducted. An oxide film having a thickness of about30 nm was formed.

The wafer subjected to annealing was then subjected to treatment forremoving an oxide film with a solution of hydrofluoric acid, and then topolishing with a stock removal of 5 μm, and then the number of COP (sizeof 0.09 μm or more) at a deep zone was measured. Measurement of COP wasconducted using SurfScan SP1 manufactured by KLA Tencor Corporation. Forcomparison, a wafer obtained by subjecting the same silicon singlecrystal wafer as above to annealing of H₂/1200° C./one hour (Comparativeexample 1) and a wafer obtained by subjecting the same wafer toannealing of Ar/1200° C./one hour (Comparative example 2) were polishedwith a stock removal of 5 μm, and the number of COP was measured in asimilar method to the above.

The result of measurement was shown in FIG. 3. As shown in FIG. 3, thenumber of COP in the wafer of Example 1 was 400 or less in a 8-inchwafer, which corresponded to COP density of 1.3/cm² or less.Accordingly, the method of the present invention had a higher grown-indefect eliminating effect compared with conventional H₂- orAr-annealing.

Oxide dielectric breakdown voltage of the wafers polished with a stockremoval of 5 μm was measured. The results were shown in FIG. 4 and FIG.5. A good chip yield of TDDB (Time Dependent Dielectric Breakdown)herein means a good chip yield when a chip having a time dependentdielectric breakdown voltage of 25 C/cm² or more measured under thecondition: gate oxide film thickness of 25 nm, gate area of 4 mm²,stress electric current of 0.01 A/cm² and room temperature, or a chiphaving a time dependent dielectric breakdown voltage of 5 C/cm² or moremeasured under the condition: gate oxide film thickness of 25 nm, gatearea of 4 mm², stress electric current of 0.01 A/cm² and 100° C., isdefined as a good chip.

A good chip yield of TZDB (Time Zero Dielectric Breakdown) herein meansa good chip yield when a chip having a time zero dielectric breakdownvoltage of 8 MV/cm or more measured under the condition: gate oxide filmthickness of 25 nm, gate area of 8 mm², 1 mA/cm² of electric currentdensity in decision at room temperature is defined as a good chip.

As shown in FIGS. 4 and 5, the wafer subjected to the heat treatment ofthe present invention had excellent oxide dielectric breakdown voltageeven in a deep zone, compared to the wafers subjected to H₂- orAr-annealing.

From the above results, it is clear that if a silicon single crystalthat is to be a bond wafer is produced according to the method of thepresent invention, the wafer having few crystal defects and excellentoxide dielectric breakdown voltage can be produced. Accordingly, if aSOI wafer is produced using such a silicon single crystal wafer, SOIwafer having few crystal defects can be obtained.

The SOI wafers having SOI layer with a thickness of about 0.1 μm wereproduced according to an ion implantation delamination method, usingthree kinds of bond wafers produced under the above-mentioned condition.Production condition was as follows.

1) Hydrogen ion implantation condition:

H+ion, implantation energy 30 keV

2) Delamination heat treatment condition:

oxidizing atmosphere, 500° C., 30 minutes

3) Bonding heat treatment condition:

nitrogen atmosphere (containing slight amount of oxygen), 1200° C., 120minutes

4) Touch polishing (polishing with slight stock removal on the surfaceof SOI) done

5) Base wafer oxide film: 300 nm

COP in SOI wafer produced above was observed according to a HF dipmethod. The HF dip method comprises dipping the SOI wafer having thethin SOI layer as above was dipped in a 50% aqueous solution of HF, ifthere is a defect penetrating the SOI layer, HF reaches BOX through itto etch the oxide film and form etch pits, and observing etch pits witha microscope through the thin SOI layer to evaluate COP in a wafer. Theresults were shown in Table 1.

TABLE 1 Heat treatment COP density atmosphere (number/cm²) Example 1Ar + Ar/O₂ 0.2 Comparative only H₂ 1.8 Example 1 Comparative only Ar 1.9Example 2

As shown in Table 1, SOI wafer of Example contains very few COPpenetrating the SOI layer, compared with conventional SOI wafersobtained by subjecting bond wafers only to H₂-annealing or Ar-annealing.As described above, the density of COP having a size of 0.09 μm or morein the SOI wafer of Example 1 was 1.3/cm² or less at any depth in theSOI layer. Namely, SOI wafer having quite excellent quality can beobtained.

Although there remained a step of about 0.2 to 0.3 μm in a peripheralpart of the bond wafer after delamination formed as byproduct when SOIwafer of Example 1 was produced, the step could be removed only byremoving the oxide film on the surface, and then polishing thedelaminated surface with a stock removal of about 1 μm, and good mirrorsurface having no exposed oxide precipitates could be obtained.Accordingly, it was confirmed that there was no problem in bonding evenwhen the wafer was used as a new bond wafer or a base wafer.

Example 1, Example 2 and Example 3

In accordance with the method of the present invention, a bond wafer ofa bonded SOI wafer was produced using three kinds of silicon singlecrystal wafer, and quality thereof was evaluated. As silicon singlecrystal wafers, the wafer used in Example 1, the wafer sliced from asingle crystal produced under the same condition as Example 1 exceptthat a pulling rate of the single crystal was 1.9 mm/min (Example 2),and the wafer sliced from a single crystal produced under the samecondition as Example 1 except that the crystal in which nitrogen wasdoped at 10¹⁴ atoms/cm³. These wafers were subjected to annealing at1200° C. for 40 minutes in an atmosphere of 100% Ar, and then annealingat the same temperature in an atmosphere of a mixed gas of 30% oxygenand 70% argon for 20 minutes. After removing an oxide film with a HFsolution, and the wafers were polished with a stock removal of 5 μm, andthen the number of COP (≧0.09 μm) at a deep zone was measured. Theresults were shown in FIG. 6.

As shown in FIG. 6, the crystal containing the least COP was the waferconsisting of the crystal in which nitrogen was doped, and the number ofCOP gets large in the order of the above-mentioned wafer, the waferpulled at high speed, and the wafer pulled at general speed.Accordingly, a bond wafer having less grown-in defects can be producedusing the crystal pulled at high speed or the crystal in which nitrogenwas doped. When the crystal was pulled at high speed, time for pullingcrystal can be shorten, and thus throughput can be improved.

As for the bond wafers produced under three kinds of condition asdescribed above, SOI wafers were produced by the same method as Example1, and COP was evaluated. The results were shown in Table 2.

TABLE 2 COP density Bond wafer (number/cm²) Example 1 General pullingrate 0.2 no nitrogen was doped Example 2 High pulling rate 0.1 nonitrogen was doped Example 3 General pulling rate  0.01 nitrogen wasdoped

As shown in Table 2, COP density of the SOI wafer produced using thesilicon single crystal wafer consisting of crystal pulled at high speedwas half of COP density of the SOI wafer produced using a generalsilicon single crystal wafer. COP density of the SOI wafer producedusing the silicon single crystal wafer consisting of crystal in whichnitrogen was doped was one twentieth of general one. Accordingly, theSOI wafer having further excellent SOI layer can be obtained using thecrystal pulled at high speed or the crystal in which nitrogen was doped.

Example 4, Comparative Example 3, Comparative Example 4

The same silicon single crystal wafer as used in Example 1 was subjectedto annealing in 100% Ar atmosphere at 1200° C. for 40 minutes, andsubsequently to oxidation in atmosphere containing water vapor at 150°C. for 240 minutes to form an oxide film having a thickness of 1.0 μm.The bonded SOI wafer having SOI layer with thickness of 5 μm and BOXlayer with thickness of 1 μm was produced according to a generalgrinding and polishing method using the wafers produced above as a bondwafer. Thickness of the oxide film was measured using MPV-SPmanufactured by Leitz Corporation.

Oxide dielectric breakdown voltage was compared as for the SOI waferproduced above, the SOI wafer subjected to annealing of H₂/1200° C./onehour (Comparative Example 3) or the wafer subjected to annealing ofAr/1200° C./one hour (Comparative Example 4), then cooled to roomtemperature, and subsequently subjected to oxidation heating treatment(oxidation in atmosphere containing water vapor at 1150° C. for 240minutes). Measurement condition was the same as Example 1.

The results were shown in FIG. 7 and FIG. 8. As shown in the results,the wafer subjected to the heat treatment of the present invention hasexcellent in TZDB and TDDB, even when the thickness of the SOI layer wasmore than 0.5 μm, namely has excellent in oxide dielectric breakdownvoltage, whereas a conventional annealing method is not effective forthe wafer having the SOI layer with the same thickness as above.

The present invention is not limited to the above-described embodiment.The above-described embodiment is a mere example, and those having thesubstantially same structure as that described in the appended claimsand providing the similar action and effects are included in the scopeof the present invention.

For example, when silicon single crystal ingot is grown according toCzochralski method whether nitrogen is doped or not, a magnetic fieldmay be applied to a melt or not. Namely, the term “a Czochralski method”includes not only general Czochralski method but also MCZ method.

Furthermore, heat treatment at high temperature in non-oxidizingatmosphere and heat treatment in oxidizing atmosphere that is essentialfeature of the present invention can be applied to any steps inprocessing of a wafer. For example, it can be applied after chemicaletching step after cutting a wafer, or after rough polishing step thatis following to the above step, or after the final polishing step or thelike.

As for the heat treatment in non-oxidizing gas atmosphere in the presentinvention, explanation in the above embodiment of the present inventionhas focused on the case that argon or nitrogen is used. However,atmosphere is not limited to argon and nitrogen, there can be used theabove-gas containing hydrogen in an amount less than explosion limit,rare gas having the same effect as argon, such as helium, neon, krypton,xenon can be used, and are included in the scope of the presentinvention.

What is claimed is:
 1. A method of producing a bonded SOI wafercomprising bonding a bond wafer and a base wafer via an oxide film andthen reducing thickness of the bond wafer, wherein a silicon singlecrystal ingot is grown according to Czochralski method, the singlecrystal ingot is then sliced to produce a silicon single crystal wafer,the silicon single crystal wafer is subjected to heat treatment in anon-oxidizing atmosphere at a temperature of 1100° C. to 1300° C. forone minute or more and continuously to a heat treatment in an oxidizingatmosphere at a temperature of 700° C. to 1300° C. for one minute ormore without cooling the wafer to a temperature less than 700° C. toprovide a silicon single crystal wafer wherein a silicon oxide film isformed on the surface, and the resultant wafer is used as the bondwafer.
 2. The method of producing the bonded SOI wafer according toclaim 1 wherein the non-oxidizing atmosphere is argon, nitrogen or amixed gas of argon and nitrogen.
 3. The method of producing the bondedSOI wafer according to claim 1 wherein the oxidizing atmosphere isatmosphere containing water vapor.
 4. The method of producing the bondedSOI wafer according to claim 1 wherein the oxidizing atmosphere is dryoxygen atmosphere or a mixed gas atmosphere of dry oxygen and argon ornitrogen.
 5. The method of producing the bonded SOI wafer according toclaim 4 wherein thickness of the oxide film formed by the heat treatmentin the oxidizing atmosphere is 20 to 100 nm.
 6. The method of producingthe bonded SOI wafer according to claim 1 wherein thickness of the oxidefilm formed by the heat treatment in the oxidizing atmosphere is 20 to100 nm.
 7. The method of producing the bonded SOI wafer according toclaim 1 wherein the oxide film is previously formed on the surface ofthe wafer before the heat treatment in a non-oxidizing atmosphere. 8.The method of producing the bonded SOI wafer according to claim 7wherein thickness of the thermal oxide film on the surface of the waferafter the above-mentioned heat treatment in the oxidizing atmosphere is300 nm or more.
 9. The method of producing the bonded SOI waferaccording to claim 1 wherein a silicon single crystal ingot is grownaccording to Czochralski method with controlling a cooling rate at 1150°C. to 1080° C. of the single crystal ingot to be 2.3° C./min or more.10. The method of producing the bonded SOI wafer according to claim 1wherein a silicon single crystal ingot in which nitrogen is doped isgrown according to Czochralski method.
 11. The method of producing thebonded SOI wafer according to claim 10 wherein silicon single crystalingot in which nitrogen is doped according to Czochralski method, andthe concentration of nitrogen doped in the single crystal ingot is1×10¹⁰ to 5×10¹⁵ atoms/cm³.
 12. The method of producing the bonded SOIwafer according to claim 1 wherein the concentration of oxygen containedin the single crystal ingot is 18 ppma or less.
 13. A bonded SOI waferproduced by the method according to claim
 1. 14. A method of producing abonded SOI wafer comprising bonding a bond wafer and a base wafer via anoxide film and then reducing thickness of the bond wafer, wherein asilicon single crystal ingot is grown according to Czochralski method,the single crystal ingot is then sliced to produce a silicon singlecrystal wafer, the silicon single crystal wafer is subjected to heattreatment in a non-oxidizing atmosphere at a temperature of 1100° C. to1300° C. for one minute or more and continuously to a heat treatment inan oxidizing atmosphere at a temperature of 700° C. to 1300° C. for oneminute or more without cooling the wafer to a temperature less than 700°C. to provide a silicon single crystal wafer wherein a silicon oxidefilm is formed on the surface, at least one of hydrogen ions and raregas ions are implanted into the surface via a silicon oxide film of thewafer to form an ion implanted layer, and the resultant wafer is used asthe bond wafer, which is then brought into close contact with the basewafer via the silicon oxide film of the bond wafer, followed bydelamination at the ion implanted layer by heat treatment.
 15. A methodof producing the bonded SOI wafer wherein the bond wafer delaminated atthe ion implanted layer in the method of producing a bonded SOI waferdescribed in claim 14 is used as a new bond wafer.
 16. A method ofproducing the bonded SOI wafer wherein the bond wafer delaminated at theion implanted layer in the method of producing a bonded SOI waferdescribed in claim 14 is used as a new base wafer.
 17. The method ofproducing the bonded SOI wafer according to claim 14 wherein thenon-oxidizing atmosphere is argon, nitrogen or a mixed gas of argon andnitrogen.
 18. The method of producing the bonded SOI wafer according toclaim 14 wherein the oxidizing atmosphere is atmosphere containing watervapor.
 19. The method of producing the bonded SOI wafer according toclaim 14 wherein the oxidizing atmosphere is dry oxygen atmosphere or amixed gas atmosphere of dry oxygen and argon or nitrogen.
 20. The methodof producing the bonded SOI wafer according to claim 19 whereinthickness of the oxide film formed by the heat treatment in theoxidizing atmosphere is 20 to 100 nm.
 21. The method of producing thebonded SOI wafer according to claim 14 wherein thickness of the oxidefilm formed by the heat treatment in the oxidizing atmosphere is 20 to100 nm.
 22. The method of producing the bonded SOI wafer according toclaim 14 wherein the oxide film is previously formed on the surface ofthe wafer before the heat treatment in a non-oxidizing atmosphere. 23.The method of producing the bonded SOI wafer according to claim 22wherein thickness of the thermal oxide film on the surface of the waferafter the above-mentioned heat treatment in the oxidizing atmosphere is300 nm or more.
 24. The method of producing the bonded SOI waferaccording to claim 14 wherein a silicon single crystal ingot is grownaccording to Czochralski method with controlling a cooling rate at 1150°C. to 1080° C. of the single crystal ingot to be 2.3° C./min or more.25. The method of producing the bonded SOI wafer according to claim 14wherein a silicon single crystal ingot in which nitrogen is doped isgrown according to Czochralski method.
 26. The method of producing thebonded SOI wafer according to claim 25 wherein silicon single crystalingot in which nitrogen is doped according to Czochralski method, andthe concentration of nitrogen doped in the single crystal ingot is1×10¹⁰ to 5×10¹⁵ atoms/cm³.
 27. The method of producing the bonded SOIwafer according to claim 14 wherein the concentration of oxygencontained in the single crystal ingot is 18 ppma or less.
 28. A bondedSOI wafer produced by the method according to claim 14.